I beg to differ. More skill? How come? Putting in numbers takes the same amount of effort not skill. I've been around before the speedtest on GT5 so I know what tuning without it means. It does make it more fun, I can't argue with that. Online is always more fun than offline. The speedtest only takes the factors that you can't influence out of tuning a car. Like internet speed (ping), the TV/Monitor and other hardware you're running, "reaction time"...
I never understood why it would take more skill to tune a car against friends, or randoms, online, because there are so many variables that can decide if your tune is fast or not. Hypothetically speaking, what if you had a better tune than me, a faster one, and when we raced, I would tree you, and you would not win. Someone else would then do the same thing to you, and you'd lose. What does that mean? Did your tune lose, or was it your tree? It takes more luck to tune a car (fast) online, not skill.
You still can tune by yourself on GT6, just like on GT5, by going against your ghost. And even that is unreliable because there is very little chance that your revs will bounce the same amount as the run before (on GT5 that was a very relavant factor on speedtest, that's why times varied a lot from run to run on the same tune).